Asymmetric Synthesis of Spirocyclic 2-Benzopyrans for Positron Emission Tomography of σ(1) Receptors in the Brain

Sharpless asymmetric dihydroxylation of styrene derivative 6 afforded chiral triols (R)-7 and (S)-7, which were cyclized with tosyl chloride in the presence of Bu(2)SnO to provide 2-benzopyrans (R)-4 and (S)-4 with high regioselectivity. The additional hydroxy moiety in the 4-position was exploited for the introduction of various substituents. Williamson ether synthesis and replacement of the Boc protective group with a benzyl moiety led to potent σ(1) ligands with high σ(1)/σ(2)-selectivity. With exception of the ethoxy derivative 16, the (R)-configured enantiomers represent eutomers with eudismic ratios of up to 29 for the ester (R)-18. The methyl ether (R)-15 represents the most potent σ(1) ligand of this series of compounds, with a K(i) value of 1.2 nM and an eudismic ratio of 7. Tosylate (R)-21 was used as precursor for the radiosynthesis of [(18)F]-(R)-20, which was available by nucleophilic substitution with K[(18)F]F K222 carbonate complex. The radiochemical yield of [(18)F]-(R)-20 was 18%–20%, the radiochemical purity greater than 97% and the specific radioactivity 175–300 GBq/µmol. Although radiometabolites were detected in plasma, urine and liver samples, radiometabolites were not found in brain samples. After 30 min, the uptake of the radiotracer in the brain was 3.4% of injected dose per gram of tissue and could be reduced by coadministration of the σ(1) antagonist haloperidol. [(18)F]-(R)-20 was able to label those regions of the brain, which were reported to have high density of σ(1) receptors.